116 F
. Buckley et al. Livestock Production Science 64 2000 107 –119
Fig. 4. Effect of cow genetic merit ———, HG; - - -, MG on condition score by week of lactation averaged over the three lactations.
20.12 vs. 20.07 of lactation when compared to and Ireland
.0.85 genetic correlation but a consid- the MG cows. The averaged live weight gain kg
erable scaling effect between the two countries. This day between first and second, and between second
scaling effect indicated that grass-based feeding and third lactation was significantly P
,0.01 higher systems like those in Ireland reduce the ability of the
11.20 vs. 10.91 for the HG cows when compared animal to exploit its full genetic advantage. In the
to the MG cows. present study there was no interaction between cow
genetic merit and feeding system for any of the performance traits measured Buckley, 1999. The
4. Discussion difference in milk production observed between the
two genetic groups, averaged across first and second 4.1. Milk yield and composition
lactation, was 931 kg. This is larger than the expected difference of 670 kg from the predicted
Holmes 1988 reviewed the results of a series of difference PD information. This suggests that
grazing studies undertaken in New Zealand with both present day high genetic merit cows can perform
Jersey and Friesian cows. In all cases the high merit satisfactorily on grass-based systems when the ani-
cows outperformed the low merit cows in terms of mals are allowed adequate access to high quality
milk production irrespective of the stocking rate. pasture.
Bryant 1984 concluded that where the objective is high milk production per ha from grazed grass, top
4.2. DM intake priority should be given to improving the genetic
merit of the dairy herd. However the high genetic Selection of cows for higher milk production leads
merit cows used in these studies would be considered to higher feed intake as a consequence of the genetic
low when compared to present day high genetic correlation between the traits Van Arendonk et al.,
merit cows. Cromie et al. 1997 investigated the 1991. Previous studies Meijs and Hoekstra, 1984
performance of daughters of Holstein Friesian sires have suggested that the upper limit of grass intake
in USA and Ireland. The results indicated that there under ideal grazing conditions is 16.9 kg DM day.
was no evidence of re-ranking of sires between USA The present study indicates that with present day
F . Buckley et al. Livestock Production Science 64 2000 107 –119
117
high genetic merit cows this upper limit can be influence on partial efficiency of ME use for milk
increased. Averaged across the three lactations, at production Blake and Custodio, 1984; Grainger et
pasture the HG cows consumed on average 1.2 kg al., 1985b. Therefore most of the remaining extra
DM cow day more than the MG cows. These results energy requirement for milk yield with HG cows has
are higher than those reported previously Grainger to come from body tissue mobilisation. The greater
et al., 1985a; Belyea and Adams, 1990; Gordon et live weight and condition score change in early
al., 1995. Total DM intake expressed in kg kg live lactation week 1 to 8 with the HG cows is similar
weight was 0.033 and 0.030 for the HGI and MGI to that observed previously Grainger et al., 1985a;
cows, respectively. These values are similar to that Veerkamp et al., 1994; Gordon et al., 1995 indicat-
reported by Veerkamp et al. 1994 for mature dairy ing that the HG cows were in a larger negative
cows selected line in the first 182 days of lactation energy balance Britt, 1992. Butler and Smith
of 0.032 and 0.029 on high and low concentrate 1989 showed that greater negative energy balance
diets, respectively. The increased area 5 to 6 in early lactation is associated with reduced re-
allocated to the HG with similar post-grazing sward productive performance. The large difference in
surface heights is in line with the increased intake condition score between the two genetic groups at all
achieved. This was also demonstrated by a large stages of lactation may be as a consequence of
grazing experiment with Jersey cows of two genetic selection for higher milk yield and angularity in HG
indexes at varying stocking rates in New Zealand cows. Veerkamp and Brotherstone 1997 observed
Bryant, 1983a, 1984, 1985. The higher SDMI similar characteristics in HG cows. The increased
observed with the HG cows during the non-lactating live weight going from first to third lactation was
period is similar to that observed previously Bryant, similar for both genetic groups. However the in-
1983b. crease in body condition score was much greater,
While at pasture the difference in DM intake e.g. for the MG
10.42 compared to the HG between the genetic groups was 0.7, 1.4 and 1.6 kg
10.07 at week 8 of lactation. This suggests that DM day in the first, second and third lactation,
cows ‘seek’ to reach a certain condition-score in respectively. On average the DM intake estimates
early-lactation, which is influenced by genotype were 4.2 kg higher in the second lactation compared
Veerkamp et al., 1994. This also supports the view to those observed in the first lactation at a similar
of Emmans and Neilson 1984 that animals reduce stage in lactation. The DM intake estimates in the
their feed intake or increase production when more present study indicate that peak DM intake under
lipid is available for mobilization, rather than the grazing conditions are achieved later in lactation than
view that animals mobilize lipid because they with indoor systems where it is estimated to occur at
produce more milk than can be supported from around week 16 of lactation Bauman and Currie,
intake alone. 1980, Fig. 2. The increased grass DM intake
associated with HG cows requires a lower stocking rate during the grazing season to allow for a higher
daily grass allowance.
5. Conclusions
